1. Field of the Invention
This invention relates to a method and apparatus for fluid catalytic cracking (FCC) of a hydrocarbon feed in a reactor having a multi-feed nozzle system. More particularly, it relates to an improved method and apparatus for individually controlling the feed of oil and steam to each of a plurality of nozzles of a multi-feed nozzle system to achieve a more uniform ratio of catalyst and oil across a cross-section of an FCC riser.
2. Discussion of the Prior Art
The field of catalytic cracking, particularly fluid catalytic cracking, has undergone significant developments due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered, requiring refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in an FCC installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is combined with hydrocarbon feed (oil) and steam and is transferred in suspended or dispersed phase condition generally upwardly through one or more conversion zones (FCC riser), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser conversions, occurring at temperatures of at least 1,000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the FCC riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst. During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion step. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by mechanical means and/or stripping gas in a separate catalyst stripping zone. Hydrocarbon conversion products separated from the catalyst and stripped materials are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
Of particular interest has been the development of methods and systems for feeding oil, steam and catalyst into an FCC riser and improving the contact between the oil and catalyst to obtain uniform mixing as they travel up the FCC riser.
Various processes and mechanical means have been employed heretofore to feed steam, oil and catalyst to an FCC riser and improve the contact of the oil and catalyst. Several of these are discussed below.
Baumann et al, U.S. Pat. Nos. 3,261,776 and 3,353,925 disclose a process and apparatus, respectively, for feeding steam and catalyst into an FCC riser, mixing these streams with hydrocarbon feed and then passing the combination up the FCC riser. A series of single Venturi constricting zones are used to improve the mixing and contact of catalyst with the hydrocarbon feed and steam.
Griffel et al, U.S. Pat. No. 3,654,140, in a first embodiment, discloses a process which combines steam and oil, and then passes them across a valve and through a Venturi nozzle, and into an FCC riser where the steam and oil mix with catalyst. In a second embodiment, steam and oil are fed through concentric pipes into an FCC riser where the steam and oil mix with catalyst. The steam passes through the annulus between the inner and outer pipe, while oil passes through the inner pipe. Steam and oil rates are controlled by valves.
Snyder, Jr., U.S. Pat. No. 3,812,029 discloses a nozzle for injecting combined steam and catalyst into an FCC riser. The nozzle comprises concentric nozzles, which include an inner nozzle for oil and an outer nozzle for steam or water.
While each of the systems described above mix a catalyst and oil feed, there still remains a need for a system which obtains, as near as possible, a uniform ratio of catalyst to oil feed across the cross-section of a riser. The present invention is directed to filling this need.